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Sir,

There is, at the present time, a good deal of interest in long-life motor oils. One such oil has been on the market for several weeks; another which has been widely available abroad for some months past has now made its appearance in this country.

You may perhaps have wondered how Castrol, whose motor oil is used by half the motorists in Britain, regard this development. Are we, for example, being unduly conservative in not bringing out a long-life oil of our own at this stage? I thought, therefore, that I would let you know, as background information, what our views on this subject are.

Technically, there is no problem in producing oils of this type. The question is whether it is better for the owner of a Ford, for example, to get rid of his old oil once every 10,000 miles or twice every 10,000 miles.

As you know, car makers recommend that oil be changed periodically not because it wears out but because of contamination and dilution—no matter how good an oil may be it cannot guard itself against dilution.

As the custodians of the mechanical well-being of several million cars, we have had to ask ourselves whether the obvious technical advantages of two fresh sumpfuls of oil in 10,000 miles (or whatever the period might be) are outweighed by the apparent economy and convenience of a single sump drain in the same period.

Economy

Does the motorist, in fact, save himself money by using a long-life oil. On the face of it, this might seem to be the case but the actual figures tell a different story.

Let me take the example of a motorist who does 10,000 miles a year (an above-average mileage) in a car the maker of which recommends a 5,000-mile oil change period. I will further suppose that the sump capacity is 8 pints (a reasonably typical figure) and that oil consumption at 1,000 miles to the pint is relatively low. This is what we find:—

Is this example loaded in favour of the “normal” oil? Far from it. Reduce the assumed oil consumption to 500 miles to the pint, the figures are then: long-life oil 92s. 2 1/2 d., “normal” oil 77s. 10d. With the exceptionally low oil consumption of 1,500 miles to the pint, long-life does certainly show a small saving—a matter of 1s. 7d. per year. Most motorists, in fact, cover less than 10,000 miles a year—the lower the annual mileage, the more economical is the use of the “normal” oil.

Try the sum with any figures you like, the result will always be in favour of the “normal” oil and the recommended change period, except in the case of cars which require very little topping up indeed between changes.

Convenience

It might be argued that some motorists would accept the extra cost of using a long-life oil because of the time they would save in not having to put the car into the garage so often. But the frequency of their visits could not be reduced because regular servicing, as specified by the manufacturer, is necessary whether the sump oil is changed at the time or not.

For these reasons, we have come to the conclusion that it would not be in the interests of the motorist for us, as oil technologists, to recommend any departure from the lubrication procedures laid down by the engineers who have designed his car.